Zooplankton distribution and dynamics in a North Pacific Eddy of coastal origin: II. Mechanisms of eddy colonization by and retention of offshore species

Mesoscale anticyclonic eddies form annually in late winter along the eastern margin of the subarctic North Pacific. Eddies that originate off the southern tip of the Queen Charlotte Islands (near 52°N 132°W) are called ‘Haida Eddies’. During the subsequent 1–3 years, they propagate westward into the Alaska Gyre. Enroute, the eddies are colonized by zooplankton originating from the central British Columbia continental shelf, the continental slope and along-slope boundary current, and the oceanic Alaska Gyre. Eddies also gradually lose kinetic energy, water properties, and biota to the surrounding ocean. In this paper, we analyze zooplankton samples from Haida eddies obtained in late winter, early summer and autumn of 2000, and in early summer and autumn of 2001, and compare the within-eddy zooplankton distributions, abundances, and community composition of the oceanic-origin species to observations from the continental margin and Alaska Gyre source regions. Most between-region comparisons were consistent with a hypothesis that the eddy zooplankton are a mixture intermediate in abundance and community composition between the BC continental margin and offshore Alaska Gyre source regions (although usually closer to the Alaska Gyre). However, about 30% of the comparisons showed within-eddy abundances higher than in either source region. This outcome cannot arise from mixing alone. Aggregation and retention appear to be linked to vertical distribution behavior: most of the successful taxa spend much of their time below the surface mixed layer. This minimizes their exposure to wash-out by three physical processes: • pwelling and surface divergence that accompanies weakening of the anticyclonic geostrophic currents. but intermittent flushing of the surface layer by Ekman transport during strong wind events. ge across the eddy margin/geostrophic streamlines caused by temporary displacement by wind-driven inertial currents.